Field of the Invention
The present invention relates to a composite substrate comprised of an alternating pattern of electrically insulating material and electrically conductive material. More specifically, a composite substrate comprises an alternating pattern of diamond and metallic portions, pieces, or segments.
Description of Related Art
Active electronic devices that include P-N junctions generate heat in operation. Examples of such active devices include a semiconductor laser, a light-emitting diode, and a laser diode. Such heat should desirably be removed promptly to avoid undesirable temperature rises in the active device which can negatively impact the temporary or long-term performance of the active device.
In an example, in connection with a laser diode, a temperature change can lead to a wavelength shift of the laser light being produced by the laser diode. Such wavelength shift, even subtle, can be undesirable. In an example, a high power laser diode can have an electrical energy to light conversion efficiency between 10% to 50%. The rest of the electrical energy is converted to heat which needs to be removed, otherwise the semiconductor junction temperature rises to an undesirable level. In addition, temperature rise due to insufficient heat removal has a direct influence on the output wavelength and bandgap. In an example, for a temperature change of every three degrees centigrade, the wavelength of a diode laser can change by nearly 1 nm. In addition, the output power of the laser diode can decrease as the temperature increases.
Typically, such active devices are coupled to a submount substrate which aids in the removal of heat via various bonding mechanisms, such as, for example, adhesive or solder. In an example, where the active device is a semiconductor laser or a laser diode, the coefficient of thermal expansion (CTE) of the materials forming such active device can range between 3×10−6 meters/meters-degree Kelvin (m/m-K) to 7×10−6 m/m-K. In contrast, the submount substrate material to which such active devices are mounted can have a CTE between 10 to 25×10−6 m/m-K, depending upon the material selected to form the submount substrate. As can be seen from this example, there is a significant mismatch between the CTE of the material forming the active device (between 3 and 7×10−6 m/m-K) and the CTE of the material forming the submount substrate (between 10 and 25×10−6 m/m-K).
It is known in the art that differences in CTE between the material of the active device and the submount substrate mentioned above can result in bonding failure between the active device and the submount substrate in response to changes in temperature of the active device during its operation. To avoid this problem, heretofore, the CTEs of the materials forming the submount substrate and the active device were chosen to be as close as possible. However, these efforts have not produced satisfactory results.
Heretofore, a real challenge existed in achieving a close CTE match between the material(s) of a submount substrate and the material(s) of an active device, while simultaneously attaining efficient heat removal from the active device via selecting a submount substrate material that was highly thermally conductive. In one example of the prior art, the CTE of a submount substrate can be tuned by making a metallic composite of copper-tungsten, copper-molybdenum, etc. The CTE of Cu—W and Cu—Mo can be tuned to between 6×10−6 m/m-K and 9×10−6 m/m-K from copper CTE of 17×10−6 m/m-K, depending on the percentage of copper level. For example, 15% copper in tungsten has a CTE of 7.2×10−6 m/m-K, while its thermal conductivity is about 210 W/m-K. In another example, 20% copper in molybdenum has a CTE of 7.5×10−6 m/m-K, while its thermal conductivity is only about 165 W/m-K.
One of the most thermally conductive materials is diamond having a thermal conductivity as high as 2,200 W/m-K or greater. Accordingly, diamond is an ideal material for heat removal from an active device. However, diamond has a CTE of about 1×10−6 m/m-K, substantially deviating from the CTE of an active device material (between 3 and 7×10−6 m/m-K). Accordingly, a temperature rise during operation of an active device mounted on a diamond submount substrate produces thermal compression stress on the active device. Such thermal stress due to CTE mismatch between the active device and the diamond submount substrate can lead to undesirable bonding failure of the active device to the diamond submount substrate.
In another example, the CTE of a submount substrate can be tuned by making a composite of diamond particles within a metal or metallic matrix, such as aluminum, copper, and/or silver. The diamond volume percentage of such composite can reach as high as 70% in such metal or metallic matrix. The thermal conductivity of such composite of diamond particles and metal or metallic matrix can range from 300 to 650 W/m-K, never reaching a theoretically thermal conductivity value of the composite (determined by a linear model by giving a volume percentage of diamond particles and the volume percentage of the metal matrix). For a composite of 70 volume percent diamond particles and 30 volume percent copper as a matrix, the theoretical thermal conductivity, using a linear model, is calculated to be about 1,320 W/m-K.
It is believed that the inability to achieve such theoretical thermal conductivity of the composite of diamond particles and metallic copper matrix can be due to voids and/or interface materials between the surface of diamond particles and the metallic matrix. In an example, direct mix and melt of diamond particles and metal or metallic matrix can lead to formation of voids on the interfaces of diamond particles and the metal or metallic matrix, resulting in a lower loading of diamond particles, which, in-turn, can lead to a lower thermal conductivity. The surface of the diamond particles can be modified with an interface material such as, for example, a layer of silicon carbide, tungsten carbide, molybdenum carbide, or other any other suitable metal carbide, which allows higher loading of diamond particles into the metal or metallic matrix. However, the thermal conductivity of such interface material is typically significantly lower than the thermal conductivity of diamond. In an example, the thermal conductivity of silicon carbide and tungsten carbide is between 100 and 225 W/m-K, respectively. Therefore, such interface material among diamond particles and metallic matrix imparts a substantial thermal resistance during the transport of the heat energy from the active device through the submount substrate.
The failure in achieving a theoretical thermal conductivity of the composite of diamond particles and metallic matrix can also be due to a mixture of heat-conduction mechanisms. Specifically, the movement of electrons via diffusion dominates the conduction of heat in a metallic matrix. Strong Sp3 carbon-carbon covalent bonds are responsible for the high thermal conductivity in diamond, via phonon dispersion along diamond lattices, even though there are no free electrons. Energy exchange between phonon conduction in the diamond material of a diamond/metal or metallic matrix and an electron diffusion in metal material of the diamond/metal or metallic matrix can fundamentally slow down the overall heat transport from an active device though a submount substrate made of a composite of diamond particles and metallic matrix.